1. Field of the Invention
The present invention relates to a conductive material used for a resistor and a sensor, which is enhanced its mechanical strength while maintaining a stable resistance ratio.
2. Description of the Related Art
Conventionally, conductive materials have been used in a gas sensor for flammable gas such as carbon monoxide and butane, which conducts the detection of the gas on the basis of a change in resistance while heating a catalyst by resistance heating. Further, in a thermistor and an oxygen sensor using a solid electrolyte, a lead for a sensor of gas such as carbon monoxide and nitrogen oxide, and a lead for a semiconductor gas sensor, conductive materials have been used, which are required to have a stable resistance at high temperatures and have enhanced mechanical strength by a solid solution such as Pt or a Pt—Rh alloy. Such conductive materials are not remarkably oxidized even in the atmosphere at high temperatures and have stable corrosion resistance.
Conductive materials to be used for the above-mentioned purposes are required to have excellent corrosion resistance and stable resistance at the temperature to be used. Such conductive materials are used in the form of a wire rod, a thin film produced by vapor deposition or sputtering, and a film obtained by printing and heating a paste or the like. In particular, when the conductive material is used as a wire rod, it is required to have mechanical strength at a certain level or higher. Further, depending upon the purpose, the conductive material is used as a wire rod with a diameter of 50 μm or less, which is required to have satisfactory workability, as well as corrosion resistance, heat resistance, and oxidation resistance. In order to satisfy those requests, Pt and a Pt—Rh alloy are used.
However, Pt has low mechanical strength, and in the case where Pt is heated in a high-temperature during the process, crystal grains become coarse. When bending is performed during the process, the crystal grains are broken from a grain boundary.
The mechanical strength may be enhanced by adding Rh or the like to Pt. However, due to the difference in vapor pressures, the composition change is occurred to vary a resistance. Thus, there is a problem that it cannot be used for the purpose in which the change in resistance is considered to be important.
In addition, Elements to be added are limited to those which are unlikely to be oxidized, and an element such as Rh which is more expensive than Pt needs to be used.
Further, in the case of enhancing the solid solution, Rh has a small effect of suppressing the coarsening of crystal grains. In the case where Rh is exposed to high temperatures of 1,500° C. or higher during the process, Rh is coarsened to an extent about the same as Pt and may be broken from the grain boundary.
Therefore, a material in which an oxide or the like is dispersed is used. However, it is difficult to form an extra fine wire of 50 μm or less from the material, and there are such problems that the material has ductility smaller than that of Pt and a Pt alloy, and the like.